Multi-channel DPSK receiver

ABSTRACT

An optical, multi-channel, Differential Phase Shift Keying (DPSK) receiver demodulates multiple Wavelength Division Multiplexed (WDM) channels using a single interferometer. This distributes expense of the interferometer over all channels of an optical signal, allowing for deployment of cost-effective, scalable, wideband, WDM DPSK systems. For example, for an 80 channel WDM link, the receiver uses a single interferometer instead of eighty interferometers and associated stabilization hardware, dramatically reducing size, weight, power, and cost. The receiver is architecturally compatible with existing interferometer technologies so previous development and qualification efforts can be leveraged. This allows for expedited technology insertion into existing optical communications networks, including terrestrial and space-based optical networks.

RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional Application,“Polarization Independent Interferometers”, 60/639,183 filed Dec. 23,2004 concurrently herewith. The entire teachings of the aboveapplication(s) are incorporated herein by reference.

GOVERNMENT SUPPORT

The invention was supported, in whole or in part, by a grant underContract No. F19628-00-C-0002 from the United States Air Force. TheGovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION

Recently, optical Differential Phase Shift Keying (DPSK) modulation hasreceived considerable attention by the telecommunications industryprimarily due to its increased sensitivity over commonly usedOn-Off-Keying (OOK) and its reduced peak power, which mitigatesnonlinear effects in fiber-optic applications. This has led todemonstrated utility in long haul applications, with experimentsconfirming more than 3 Tb/s capacity using 80 Wavelength DivisionMultiplexed (WDM) DPSK channels. It is expected that the first widebandtelecommunications fiber optic links using WDM DPSK will be deployed by2006. DPSK is also an attractive modulation format for high-ratespectrally efficient Free Space Optical (FSO) links because the increasein sensitivity over OOK allows for a corresponding reduction in costlytransmitter power.

The benefits of optical DPSK come at the cost of increased complexity,requiring a phase modulator in the transmitter and an opticalinterferometer and balanced detection in the receiver in order to derivemaximum benefit. Of these elements, the interferometer is the mosttechnically challenging and the least mature. Control hardware isnecessary to ensure stable operation, which requires that the arms ofthe interferometer be stable to small fractions of a wavelength. As aresult, carefully designed thermo-mechanical packaging is necessary inaddition to stabilization electronics, adding to size, weight, power,and cost.

SUMMARY OF THE INVENTION

An optical, multi-channel, Differential Phase Shift Keying (DPSK)receiver, and corresponding method, employing the principles of thepresent invention demodulates multiple Wavelength Division Multiplexed(WDM) channels using a single interferometer. A DPSK receiver using asingle interferometer for WDM demultiplexing is achieved by constrainingthe received wavelength spacing (Δν_(ch)) and leveraging theinterferometer's periodic transfer function to perform demodulation onall channels (λ's). This distributes the expense of the interferometerover all channels, allowing for deployment of cost-effective, scalable,wideband, WDM DPSK systems. For example, for an 80 channel receiver,size, weight, and power (SWAP), and costs are significantly reducedthrough use of a single interferometer instead of eighty interferometersand associated stabilization electronics. The receiver isarchitecturally compatible with existing interferometer technologies, soprevious development and qualification efforts can be leveraged. Suchleveraging allows for expedited technology insertion into existingoptical communications networks, including terrestrial and space-basedoptical networks.

Accordingly, one embodiment of an optical receiver or correspondingmethod for demodulating optical signal(s) having DPSK channels accordingto the principles of the present invention includes a delay lineinterferometer that (i) demodulates optical signal(s) having DPSKchannels to optical signal(s) having channels modulated in intensity,and (ii) outputs the demodulated optical signal(s) onto at least onemain output optical path. The receiver also includes channel selectors,such as optical filters or Wavelength Division de-Multiplexers (WDMs),in the main output optical path(s) directing channels onto tributaryoptical paths. The channels may be predefined wavelengths or wavelengthranges (e.g, 1557.0 nm+/−2.0 nm). The receiver may includeoptoelectronic converters in the tributary optical paths that convertthe demodulated optical signal into respective, corresponding,electrical signals.

The delay line interferometer may be a one-bit delay lineinterferometer, a multiple bit delay line interferometer, or selectablyadjustable delay line interferometer to interfere optical signal pulsesoffset by selectable numbers. The interferometer may also include anelectronically tunable phase shifter for stabilization and/or forreceiving channels of different wavelengths.

The interferometer may be adapted to demodulate optical signals havingwavelength spacing between carrier wavelengths defining the channelswith an integer multiple of a channel rate. For example, for wavelengthson a 100 GHz International Telecommunications Union (ITU) grid (i.e.,100 GHz channel separation), standard Synchronous Optical Network(SONET) rates of 2.5 Gbps and 10 Gbps, for instance, factor evenly intothe 100 GHz spacing. Therefore, these rates are compatible with thismulti-wavelength DPSK receiver design. In another example, 40 Gbpschannel rates requires 200 GHz channel spacing, which is also compatiblewith the ITU grid and the multi-wavelength DPSK receiver.

The interferometer may also be adapted to demodulate optical signalshaving wavelength spacing between carrier wavelengths defining thechannels evenly divisible by an odd number of half channel rates of theoptical signal. For example, 40 Gbps goes into 100 GHz channel spacingexactly 2½times. The received Signal-to-Noise Ratio (SNR) remainsintact, but the received data is inverted—a condition that can beanticipated or detected and corrected via post processing. In oneembodiment, detection electronics perform the post processing bycorrecting polarity of electrical signals of received optical channelsas necessary, which may correspond to predefined optical channels. Inanother embodiment, optical elements perform the post processing bycorrecting polarity of the phase demodulated optical signal. With thiscapability, a multi-wavelength DPSK receiver can receive all channelswithout any penalty whenever the channel spacing is evenly divisible bythe half channel rate.

For prior art, single channel, DPSK receivers, the interferometer maytrack on the single carrier wavelength to optimize demodulatorperformance and compensate for transmitter or interferometer drift.However, for multiple-channel reception with a single interferometer,the interferometer can only adjust the period (i.e., wavelength orchannel separation) and/or shift the entire comb of channels. Thus, asingle interferometer cannot track and lock onto arbitrarily spacedwavelengths simultaneously in order to optimize performance. Therefore,the optical receiver according to the principles of the presentinvention may also include a feedback processor that generates signalstransmitted to transmitter(s) of the optical signal(s) to cause thetransmitter(s) to tune carrier wavelengths defining the channels. Tuningthe carrier wavelengths allows for independent channel adjustment andalignment to the interferometer for improved demodulation performance.With this feedback, independent transmitter wavelengths can be preciselyaligned to the interferometer, despite potential drift in thetransmitter wavelengths relative to the interferometer. Such feedback,for example, could be used to compensate for laser aging or for Dopplershifts.

In one embodiment, the optical receiver includes a low noise opticalamplifier that receives the optical signal(s) and outputs the received,amplified, optical signal(s) to the interferometer, where “opticalsignal(s)” in this case refers to the collection of one or more WDMchannels being received. The low noise optical amplifier may be anErbium Doped Fiber Amplifier (EDFA) or other optical amplifier known inthe art.

To support interferometer control, the interferometer may receive apilot signal or plurality of pilot signals that serve(s) as referencewavelengths that the interferometer can lock-on to for stabilizationand/or alignment to incoming optical signal(s). The pilot signal(s) maybe locally resident or distributed, may be tunable, and may be selectedto be outside the standard communications band so as not to impact theavailable communications spectrum. As an example, the pilot tone towhich the interferometer locks-on can be a dominant incoming channelthat may be selectable by the network. With feedback, the interferometerand all other channels including locally resident references can bespaced at wavelengths relative to the dominant signal. In the event thedominant channel drops out, locally resident pilot tone(s) or otherincoming channels, which have been aligned relative to the dominantchannel can take over as the reference, allowing for continuedwavelength alignment and control. The use of multiple pilot tones can beused to improve interferometer control and provide a means of measuringinterferometer parameters, such as Free Spectral Range (FSR).

The receiver may be used in various applications, such as an opticalregenerator. The receiver may also be used in various optical networkingenvironments, such as Free Space Optic (FSO) and fiber opticenvironments. Within those environments, the networks may bepoint-to-point networks, mesh networks, ring networks, broadcastnetworks, multi-access networks, and so forth.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIGS. 1A and 1B are a schematic diagrams of an optical, multi-channel,Differential Phase Shift Keying (DPSK) receiver according to theprinciples of the present invention;

FIG. 2A is a schematic diagram of an example optical delay lineinterferometer that may be used in the receiver of FIGS. 1A or 1B;

FIG. 2B is a plot of sensitivity as a function of normalized carrierfrequency offset (e.g., center wavelength) for the interferometer ofFIG. 2A and receiver of FIG. 1A or 1B;

FIG. 3A is a spectral diagram indicating channel wavelength spacing asan integer multiple of a channel rate that optimizes use of oneembodiment of the receiver of FIG. 1A or 1B;

FIG. 3B is a spectral diagram indicating channel wavelength spacing asan integer multiple of a half channel rate that optimizes use of analternative embodiment of the receiver of FIG. 1A or 1B;

FIG. 3C is a spectral diagram indicating targeted wavelength spacing onthe ITU grid that optimizes use of an alternative embodiment of thereceiver of FIG. 1A or 1B, illustrating the use of a pilot tone to lockthe interferometer to a known reference and wavelength measurement togenerate corrective feedback to transmitters of a DPSK optical signal;

FIG. 4 is a schematic diagram of network nodes employing the receiver ofFIG. 1A or 1B;

FIG. 5A is an illustration of a Free Space Optic (FSO) application inwhich the receiver of FIG. 1A or 1B is deployed in mobile platforms suchas satellites or aircraft; and

FIG. 5B is a network diagram of an optical communications networkapplication in which the receiver of FIG. 1A or 1B is deployed innetwork nodes.

DETAILED DESCRIPTION OF THE INVENTION

A description of preferred embodiments of the invention follows.

FIG. 1A and FIG. 1B are schematic diagrams of example embodiments of anoptical, multi-channel, DPSK receiver 100 according to the principles ofthe present invention. The receiver 100 receives optical signal(s) 105from optical transmitter(s) 90. The optical signal(s) 105 may have manyDPSK channels defined by distinct wavelengths, λ₁, λ₂, . . . , λ_(n). Inone embodiment, the optical signal(s) 105 are received by an opticalamplifier 110, such as a low noise Erbium Doped Fiber Amplifier (EDFA)110, that may be separate from the receiver 100 or integrated into thereceiver 100. The EDFA 110 outputs amplified optical signal(s) 105 to afirst optical splitter 115, which directs a portion of the amplifiedoptical signal(s) 105 to an interferometer 120, such as a 1-bit delayline interferometer 120, via a primary input line 118. The delay lineinterferometer 120 is sometimes referred to as a “delay and multiplydemodulator.” The remaining portion of the amplified optical signal(s)105 is directed to a wavemeter 170. The receiver 100 may leverage theperiodic transfer functions (cos² or sin²) from either arm of theinterferometer 120 to perform a delay line (e.g., one bit) demodulationon all channels (λ's) with a single interferometer. Such leveraging ispreferably performed by defining the wavelength spacing (Δν_(ch)) to bean integer multiple of a channel rate (R).Δν_(ch)=mR, where m is a positive integer.  (Equation 1)

The interferometer 120 demodulates the optical signal(s) 105 byinterfering the received optical signal pulses from each channel with anoffset version of itself, where the offset is equal to theinterferometer delay. The delay is usually a one bit delay (i.e.,interfering adjacent signal pulses), but more generally can be aninteger multiple of the one bit delay duration (i.e., interferingnon-adjacent pulses). The interference converts the optical signal ineach channel from being differentially phase modulated to beingintensity modulated. An example of an interferometer that may be used inthe receiver 100 is described below in reference to FIG. 2A.

Continuing to refer to FIG. 1, the interferometer 120 outputs aprocessed form of the optical signal(s) 105 onto first and second mainoptical paths 132-a and 132-b, respectively (collectively, optical paths132). The demodulated optical signal(s) on the two main optical paths132 are complementary; if the optical signal pulses in each channelconstructively interfere (i.e., they are in-phase), the interferometer120 directs the phase demodulated optical signals onto one main opticalpath (132-a), and if the optical signal pulses destructively interfere(i.e., they are out-of-phase), the interferometer 120 directs the phasedemodulated optical signal onto the other main optical path (132-b).

The optical paths 132 traverse a channel selector network 130 shown as1×(n+1) Wavelength Division de-Multiplexers (WDMs) 131-a, 131-b(collectively 131) (e.g., prisms, diffraction gratings, or ArrayedWaveguide Gratings (AWGs) in FIG. 1A, and alternatively defined by aseries of cascaded fiber Bragg grating (FBG) WDM filter pairs 133-1a/133-1 b, 133-2 a/133-2 b, . . . , 133-na/133-nb (collectively, WDMfilters 133) in FIG. 1B. The WDMs 131 (FIG. 1A) or filter pairs 133(FIG. 1B) is/are coupled to respective tributary optical paths 140-1a/140-1 b, 140-2 a/140-2 b, . . . , 140-na/140-nb(collectively,tributary optical paths 140).

The tributary paths for each channel are sent to post processingelements 160, which may be all optical, or optoelectronic, whichincludes elements such as communications electronics (not shown),balanced detection hardware 150, and detection electronics 155, asillustrated in FIG. 1B. The communications electronics pass networktraffic to network communications systems (FIGS. 4, 5A, and 5B). Thedetection electronics 155 may include peak RF power detection (forsignal-to-interferometer alignment), clock recovery, Forward ErrorCorrection (FEC) decoding hardware, and so forth. As shown in FIG. 1A,each of the post processing elements 160-1, 160-2, . . . 160-n,(collectively 160) may send channel performance information or metrics,such as Bit Error Rate (BER) and peak RF power, to a control processor125 via bus lines 165-1, 165-2, . . . , 165-n (collectively 165). Otherinputs to the control processor 125 may include measurements by awavemeter 170 of a pilot tone 108 and incoming optical signal(s)wavelength information, which can be used along with the BER and peak RFpower, among other information or metrics, to determine feedbacksignal(s) 185 and 188. The feedback signals 185 and 188 are communicatedfrom the control processor 125 to the pilot tone generator 107 andoptical transmitter(s) 90 via feedback paths 190 and 195, respectively.The feedback signals 185 and 188 are generated to control the pilot tone108 and incoming received signal wavelengths, respectively.

For control purposes, an optical pilot tone generator 107 generates anoptical pilot or reference tone 108 at wavelength λ_(p) that may beinjected into the interferometer 120 via a secondary input line 119 atan output of a second optical splitter 116. A portion of the pilot tone108 may also be directed via the optical splitter 116 to the wavemeter170 for wavelength measurement. Pilot tone outputs from theinterferometer 120 are directed through the channel selector network130. The a-side and b-side WDM pilot tone outputs on tributary opticalpaths 140 p-a and 140 p-b, respectively (collectively 140 p), aredirected to a control processor 125, which may measure a contrastbetween the pilot tone outputs on tributary optical paths 140 p. Themeasured contrast (D), which is the normalized difference between thepilot tone outputs on the tributary optical paths 140 p:D=(140p-a−140p-b)/(140p-a+140p-b),is a function of the pilot tone wavelength λ_(p) (or equivalently centerfrequency) and the interferometer bias (e.g., interferometer phase),which is discussed in further detail below in reference to FIGS. 2A and2B. For a given pilot tone wavelength, which may either be known ormeasured, the pilot tone contrast ratio is a function of theinterferometer bias. Therefore, sending pilot tone outputs on thetributary optical paths 140 p to the control processor 125 enables thepilot contrast to be determined, which can provide feedback parametersthat may be used to measure and the control interferometer 120, asdiscussed above.

Referring to FIG. 1B, each of the tributary optical paths 140 undergobalanced detection by balanced detectors 150, the output of which iselectrically coupled to detection electronics 155-1, 155-2, . . . ,155-n (collectively 155). Balanced detection can be accomplished in manyways, for example, using balanced detector pairs that output adifference photocurrent directly or using discrete photodetectors thatoutput their respective photocurrents to electronic elements, such as adifferential amplifier, which subsequently performs the differencing.For wavelengths on the International Telecommunications Union (ITU) grid(e.g., 100 GHz channel separation), Time Division Multiplexed (TDM)architecture standard Synchronous Optical Network (SONET) rates of 2.5Gbps and 10 Gbps, for instance, factor evenly into the 100 GHz ITUspacing. Therefore, these standard rates and wavelength spacing arecompatible with this multi-wavelength DPSK receiver design. In anotherexample, 40 Gbps channel rates requires 200 GHz channel spacing, whichis also compatible with the interferometer 120 in the optical receiver100 and the ITU grid.

If, however, the channel spacing is evenly divisible by an odd number ofhalf channel rates, for example, 40 Gbps goes into 100 GHz channelspacing exactly 2½ times, the received Signal-to-Noise Ratio (SNR)remains intact, but the received data is inverted on every other channel(see also FIG. 3)—a condition that can be anticipated or detected andcorrected via a polarity corrector 162-1 (e.g., a conditional inverter)in the post processing electronics 160 or optically inverted prior todetection by suitable optical element(s) 162-2, 162-3. An example ofsuch an optical element is a controllable delay line interferometer inwhich the relative optical phase between the two arms can be switched byapproximately a half-wavelength of the carrier frequency, or oddmultiples thereof. The interferometer 120, for instance, could act as anoptical DPSK inverter in this manner, but since it is processing allchannels simultaneously, it cannot perform polarity correctionselectively. With polarity correction capability, a multi-wavelengthDPSK receiver can receive all channels without any penalty whenever thechannel spacing is evenly divisible by the half channel rate (R/2),shown in the equation below.Δν_(ch) =mR/2, with polarity correction (m is a positiveinteger).  (Equation 2)

FIG. 2A is a schematic diagram of a representative delay lineinterferometer 120 composed of a first 50% optical splitter 210 thataccepts primary and secondary inputs 118, 119 and splits them equallybetween two internal paths or “arms” 205, 206 with relative time delayτ. The optical signals in the two paths 205, 206 are recombined with asecond 50% optical splitter 220. The interferometer 120 (i) may beconstructed as a Mach-Zehnder (as shown) or Michelson interferometer,(ii) may be waveguide, free-space, or fiber based, and/or (iii) mayinclude Faraday rotator elements to achieve polarization independence.

FIG. 2B graphically illustrates signal loss for wavelength misalignment(i.e., the carrier wavelength is offset from the optimum wavelength)given polarity correction. The transfer functions of the twointerferometer output arms 132 a, 132 b are periodic and complementary,following a cos²(π(Δf+Δφ)/FSR) and sin²(π(Δf+Δφ)/FSR) dependence,respectively. The biasing term Δφ is a measure of the relative opticalphase between the internal interferometer arms and is typicallycontrolled to maximize the signal interference in one or both of theoutput arms 132 a, 132 b. When the bias Δφ=0, the term Δf is thefrequency deviation from optimum alignment of the incoming signal to theinterferometer, which is periodic. FSR is the interferometer FreeSpectral Range, which is the interferometer spectral period, the inverseof the interferometer time delay (τ).

The theoretical contrast between the two output arms 132 a, 132 b iscalculated according to the following equation:D=cos²(π(Δf+Δφ)/FSR)−sin²(π(Δf+Δφ)/FSR)=cos(2πΔf/FSR). Thesignal-to-noise ratio (SNR) of the interferometer 120 output isdependent on the contrast. When the bias Δφ=0, the contrast and SNR goto zero when Δf=FSR/4 (or odd multiples thereof), corresponding to the 3dB point of the transfer function of both arms. Beyond this point, thedata starts to invert.

The signal loss is represented as the solid line with diamonds; thesolid line represents the interferometer cos²( )transfer function; thedashed line represents the interferometer sin²( ) transfer function. Aperformance penalty is incurred with the multi-channel DPSK receiver 100whenever the condition of Equation 2 presented above is not met since itis not possible for all channels to align to the interferometersimultaneously. This can occur, for example, when commonly used7%—overhead G.709 compliant Forward Error Correction (FEC) coding isused with standard SONET rates, which brings a 10 Gbps SONET data rateto a 10.7 Gbps coded channel rate. These rates do not factor evenly intothe 100 GHz ITU grid spacing, so it is impossible for all of the ITUgrid-based WDM channels to align with the periodicity of theinterferometer 120.

FIG. 3A is a graphical representation of a channel spacing evenlydivisible by an integer multiple of a channel rate, superimposed on theinterferometer transfer function for one output arm, (e.g., cos²). Inthis case, λ₁, λ₂, and λ_(n) are spaced by integer multiples of thechannel rate.

FIG. 3B is a graphical representation of a channel spacing evenlydivisible by the half channel rate, superimposed on the interferometertransfer function for both output arms, (i.e., cos² and sin²). In thiscase, λ₁, and λ_(n) are at an odd half channel rate spacing multiple(requiring polarity correction), and λ₂ is spaced by both integer andhalf rate multiples of the channel rate.

FIG. 3C illustrates the use of the pilot tone to lock the interferometerto a known reference. FIG. 3C also illustrates the generation offeedback signals that can be used to tune transmitted channels λ₁, λ₂and λ_(n) to the desired wavelengths, in this case aligned to theinterferometer on ITU grid channels 57, 58, and 59. A 40 GHz FSRinterferometer is used to receive, for example, 40 Gbps DPSK data on the100 GHz ITU grid. As in FIG. 3B, λ₁ and λ_(n) are at an odd half channelrate spacing multiple being located on an inverted fringe (requiringpolarity correction), and λ₂ is spaced by both integer and half ratemultiples of the channel rate. The pilot tone is set to 195,970.0 GHz,and the interferometer phase bias is controlled using a phase shifter(not shown) so that the measured normalized contrast D(λ_(p), Δφ) forthe given pilot tone wavelength is adjusted to a target value, i.e.,D(λ_(p), Δφ)=T, where {−1≦T≦1}.

Continuing to refer to FIG. 3C, the target contrast is zero, whichplaces an inverted fringe precisely on ITU channel #59 (195,900 GHz)where λ₁ is expected. Similarly, a positive fringe is aligned to ITUchannel #58 (195,800 GHz) where λ₂ is expected, and another invertedfringe is aligned to ITU channel #57 (195,700 GHz) where λ₃ is expected.Note that the polarity of the ITU channels received by theinterferometer could be inverted by simply shifting the pilot tone anodd multiple of FSR/2, (e.g., λ_(p)=195,950.0, 195,910.0, or 195,870.0).

Any of the methods known in the prior art, such as proportional feedbackcontrol, can be used to control the pilot tone 108. For example, anupdated error term can be defined as the difference between the measuredvalue of D and the target T, i.e., E_(i+1)=D_(i)−T. The phase Δφof theinterferometer, controlled by a phase shifter, can be augmented by anincrement that is proportional to the error, i.e.,Δφ_(i+1)=Δφ_(i)+g*E_(i), where g is an appropriately chosenproportionality constant. The phase of the interferometer 120 may beiteratively updated until the phase error reaches an acceptably smalllevel and converges in a stable manner to a unique phase, at whichpoint, the interferometer 120 is “locked” to the target contrast D and acorresponding phase, Δφ.

As indicated in FIG. 3C, the incoming channel wavelengths are measuredby wavemeter 170 to be 195,902.2, 195,800.0, and 195,698.2 GHz for λ₁,λ₂, and λ_(n), respectively. The control processor 125 calculates thewavelength error for each channel, which corresponds to wavelength errorΔ's of 4.2 GHz, 0.0 GHz, and −1.8 GHz for λ₁, λ₂, and λ_(n). The controlprocessor 125 sends accurate wavelength error correction information 188back to the transmitters 90 to achieve rapid convergence to optimum linkperformance. Note that error information may be useful for calibratinglocal wavelengths and measurement hardware, such as the local pilot tonegenerator 107 or wavemeter 170. For instance, if all the incomingchannels yield the same error (e.g., Δ₁ through Δ_(n)=4.5 GHz), this canbe an indicator that either the pilot tone generator 107 or thewavemeter 170 may need calibration. Alternatively, rather than correctall the transmitters with a common wavelength error, the controlprocessor 125 may shift the pilot tone 108 by 4.5 GHz to compensate forthe error common to all the transmitters.

To avoid performance penalties when using the multi-channel DPSKreceiver 100, the half channel rate and channel spacing can be forced tofactor evenly (upholding Equation 2) by either: (i) adjusting thechannel spacing to be a multiple of the channel rate, abandoning, forexample, a standard such as the ITU grid if necessary, or (ii) adjustingthe half channel rate to be an even factor of the channel spacing,abandoning, for example, SONET or G.709 standards, if necessary. Notethat applying strong Rate ½ codes to SONET data rates yields channelrates consistent with the multi-wavelength DPSK receiver and standardITU grid channel spacing.

When misalignment cannot be avoided, e.g., if conforming with existingstandards is a priority, performance penalties can be constrained to anacceptable level by operating in a regime where the incoming channelwavelength and the interferometer alignment are close enough so that themisalignment penalty or conformance concerns become negligible.

For instance, a 10.7 Gbps channel rate can be received by a 10.7 GHz FSRinterferometer, which can accept optical center frequencies every 5.35GHz (assuming the ability to anticipate or detect and correct inverteddata). While most of the interferometer fringes do not align exactly tothe 100 GHz ITU grid, none of the fringes are more than 5.35/2 or ˜2.7GHz from the ITU grid, with the average deviation of ˜1.4 GHz. Note thatif a smaller deviation is required, the deviation can be reduced by afactor of n by using Non-Adjacent (NA)-DPSK with an n NA-pulseseparation, which corresponds to an interferometer FSR narrowed by afactor of n. Thus, for 10.7 Gbps and n=2, transmitted wavelengths can betuned to align precisely with the interferometer 120, providingpenalty-free performance while maintaining a sub-GHz average deviationfrom the ITU grid. Note that for transmission of harmonically relatedchannel rates (e.g., 2.5 Gbps, 10 Gbps, and 40 Gbps), a singleinterferometer 120 with delay equal to an integer multiple of the lowestchannel rate bit period may also be used to demodulate multi-rateoptical signals (e.g., n-NA-DPSK channels with varying n). In thismanner, a single interferometer may used to demodulate multiple WDMchannels having multiple rates simultaneously. For example, a singleinterferometer with a 400 psec delay can be used to demodulate multipleoptical DPSK signals with ITU grid compliant 100 GHz spaced carrierwavelengths carrying data at 2.5 Gbps, 10 Gbps, and/or 40 Gbps rates.

FIG. 4 is a schematic diagram of a two node network 400 that includestwo network nodes 405-1 and 405-2 (collectively, network nodes 405).Each of these network nodes 405 includes transmitter(s) 90 and areceiver 100, as described in reference to FIGS. 1A and 1B. The networknodes 405 communicate via optical paths 410, which may be free space orfiber optic optical paths.

In operation, the transmitter(s) 90 transmits WDM DPSK signal(s) 105 viathe optical path 410 to a receiver 100. Proper wavelength spacing isachieved in this embodiment optionally through use of a feedback signal188, which is fed back by the receiver 100 to the transmitter 90. Thefeedback signal 188 may be determined through use of the wavemeter 170,which measures the wavelength of each channel (e.g., 1555.000 nm vs.1554.800 nm). In another embodiment, a reference or pilot tone 108 maybe temporarily or continuously introduced into the interferometer 120.The wavelength of the reference or pilot tone 108 may be known inadvance or measured with the wavemeter 170. In such an embodiment, thereference or pilot tone 108 can be used for stabilization or to lock theinterferometer 120 to a preselected comb of wavelength(s) at whichincoming channel(s) are expected to be.

Given knowledge of (i) the pilot tone wavelength, (ii) theinterferometer FSR, and (iii) the targeted locking position within theFSR, which are parameters that can be known or measured locally at thereceiver 100, the position of the entire comb of interferometer channels(i.e., location of periodic interferometer peaks and troughs) can bedetermined. This information may be relayed back to the transmitter(s)as target wavelength information or may be combined with measurement ofincoming wavelengths to generate an error signal indicating the errorbetween the received and targeted wavelength for each channel.

The error or a representation of the error is fed back to thetransmitter(s) 90 of the optical signal(s) for correction of the givenchannel's wavelength. Optionally, the error signal(s) may be used toalign the interferometer 120 to the incoming channels, therebyminimizing the aggregate error. As an example, this capability may beparticularly beneficial if all incoming wavelengths deviate from theoptimum by the same amount, a condition that can occur whenever theincoming wavelengths are (properly) spaced at an integer multiple of theinterferometer FSR. In this case, it may be preferable to tune theinterferometer 120 to the incoming signals rather than tuning all theincoming signals to the interferometer 120.

In yet another embodiment, other information available to the receivermay be used to determine the contents of the feedback signal. Examplesof such information includes (i) a bit error rate (BER) or other metricsassociated with detection of the optical signal(s) 105, such as FECbased error rate estimates or Doppler shift(s) that can be measured, forexample, via the wavemeter 170 (optically) or via clock recoveryoffset(s), or (ii) overhead or data contained in the optical signal(s)105. In such an embodiment, the transmitter(s) 90 of the opticalsignal(s) 105 may “step around” the wavelength of the channel(s) until aminimum bit error rate, for example, is found. Other min/max searchtechniques known in the art may also be employed.

The feedback signals 185, 188 (FIG. 1A) may be implemented in manydifferent forms. For example, the feedback signals 185, 188 may beanalog, digital, or packetized. The feedback signals may be communicatedvia electrical, Radio Frequency (RF), or optical paths using applicablecommunications techniques and protocols. For example, the feedbacksignals 185, 188 may be handled via a network-level maintenance andcontrol channel, traffic channel, or other suitable communicationschannel. The feedback signals 185, 188 may be a command signal orinformational signal, depending on the processing capability associatedwith the transmitter(s) 90. The feedback signals 185, 188 may beincluded in overhead or payload portions of communications packets. Thefeedback signals 185, 188 may be transmitted over the opticalcommunications path 410, optical maintenance path (not shown),electrical communications path (not shown), electrical maintenance path(not shown), or other path(s) configured for transporting the feedbacksignals 185, 188. Since optical transmitter wavelength(s) are generallystable (i.e., have a slow drift rate), correcting the transmittedwavelength(s) can be done relatively infrequently (i.e., every fewminutes, hours, days, weeks, months, or years) depending on thetransmitter 90 and possibly environmental conditions at thetransmitter(s) 90, receiver 100, or optical paths therebetween and besent from the control processor 125 on a periodic, aperiodic, eventdriven, or request driven basis.

FIG. 5A is an example communications network 500 in which the receiver100 may be employed. The communications network 500 is a Free SpaceOptic (FSO) network. The optical signal 105 is transmitted by thetransmitters 90 between mobile platforms, such as communicationssatellites 501-1 and 501-2, and received by the DPSK receiver(s) 100 ina manner as described above. The optical receiver 100 can also beemployed by other platforms, such as aircraft, and relatively stablelinks, such as building-to-building and ship-to-shore FSO links.

FIG. 5B is another example of a network in which the DPSK receiver 100may be employed. The optical network 515 includes optical nodes 405-1,405-2, 405-3, and 405-4 (collectively, nodes 405). Between each of thesenodes 405 are fiber optic or FSO communications paths 410. The opticalcommunications network 515 is configured as a Unidirectional PathSwitched Ring (UPSR) or Bi-directional Line Switched Ring (BLSR) thatmay be optically coupled to other rings or optical networks having otherconfigurations.

It should be understood that there are many other optical communicationsapplications in which the WDM receiver 100 may be employed and provideadvantages as described above.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

For example, the interferometer 120 of FIG. 1 can be more than a 1-bitdelay line interferometer. It may be a multiple bit (multi-bit) delayline interferometer in one embodiment or a dynamically selectable bitdelay line interferometer in another embodiment. The pilot tonereference wavelength of FIG. 1A, rather than being local, can beselected as one of the incoming channels. The interferometer 120 andoptical channels 105 can align to wavelengths relative to the selectedchannel through feedback from the control processor 125.

The main optical paths 132 (FIGS. 1A and 1B) may also be a single mainoptical path instead of the two main optical paths. In such anembodiment, the single main optical path may carry a signal when theinterference is constructive and no signal when the interference isdestructive, or vice-versa. Because the two main optical path embodimentprovides 3 dB more sensitivity than the single main optical pathembodiment, the two main optical path embodiment is generallypreferable. However, the single main path embodiment may be easier toimplement since it does not require both a-side and b-side elements(such as WDMs 131), balanced detection, or amplitude and time alignmentof received photocurrents for each channel.

The main optical paths 132 may also include more than two main opticalpaths. Such an embodiment may be used in cases where the interferometerinterferes more than two optical signal pulses in the DPSK channels. Forexample, future noise reduction or security schemes may facilitate orwarrant higher order DPSK demodulation, in which case, three, four, ormore main optical paths 132 may be employed.

In FIGS. 1A and 1B, instead of having a tributary optical path for eachwavelength, multiple wavelengths may be directed onto the same tributaryoptical paths and separated onto yet other tributary optical paths torespective optical receivers for processing.

In addition to use in the point-to-point network of FIG. 5A and the ringnetwork of FIG. 5B, the receiver 100 may be used in other networkconfigurations, such as mesh networks, Bi-directional Line Switched Ring(BLSR) networks, broadcast networks, or multi-access networks.

1. An apparatus for demodulating an optical signal having DifferentiallyEncoded Phase Shift Keying (DPSK) channels, the apparatus comprising: adelay line interferometer to (i) delay at least one channel in anoptical signal by multiple bits, (ii) demodulate the optical signal fromhaving DPSK channels to an optical signal having channels modulated inintensity and (iii) output the demodulated optical signal onto at leastone main output optical path; and channel selectors in the at least onemain output optical path to direct channels onto tributary opticalpaths.
 2. The apparatus according to claim 1 further includingoptoelectronic converters in the tributary optical paths to convert thedemodulated optical signal into respective, corresponding, electricalsignals.
 3. The apparatus according to claim 1 wherein the delay lineinterferometer delays at least one channel by at least one bit.
 4. Theapparatus according to claim 1 wherein the delay line interferometer isselectably adjustable to interfere optical signal pulses offset by aselectable number.
 5. The apparatus according to claim 1 wherein theinterferometer includes an electronically tunable phase shifter toreceive incoming wavelengths.
 6. The apparatus according to claim 1wherein wavelength spacing between carrier wavelengths defining thechannels is an integer multiple of a channel rate of the optical signal.7. The apparatus according to claim 1 wherein wavelength spacing betweencarrier wavelengths defining the channels is evenly divisible by an oddnumber of half channel rates of the optical signal.
 8. The apparatusaccording to claim 7 further including a polarity corrector to correctpolarity of the channels.
 9. The apparatus according to claim 8 whereinthe polarity corrector is configured to correct polarity of electricalsignals corresponding to the channels.
 10. The apparatus according toclaim 8 wherein the polarity corrector is configured to opticallycorrect polarity of the channels.
 11. The apparatus according to claim 1wherein channel rates are integer multiples of interferometer delay. 12.The apparatus according to claim 1 further including a feedbackprocessor to generate signal(s) transmitted to a transmitter(s) of theoptical signal(s) to cause the transmitter to tune carrier wavelengthsdefining the channels for adjusting wavelength position or channelseparation.
 13. The apparatus according to claim 1 further including alow noise optical amplifier to receive the optical signal and outputtingthe received amplified optical signal to the interferometer.
 14. Theapparatus according to claim 1 wherein the channel selectors includeoptical filters.
 15. The apparatus according to claim 1 wherein theinterferometer is configured to receive at least one pilot signal toallow for stabilization and control of the interferometer and wavelengthalignment of the incoming optical signal(s).
 16. The apparatus accordingto claim 1 used in an optical regenerator.
 17. The apparatus accordingto claim 1 used in a free space optic communications network.
 18. Theapparatus according to claim 1 used in a fiber optic communicationsnetwork.
 19. A method of demodulating an optical signal havingDifferentially Encoded Phase Shift Keying (DPSK) channels, the methodcomprising: delaying first optical signal pulses in at least one DPSKchannel of the optical signal by a multiple bit duration to align thefirst optical signal pulses with second differentially encoded opticalsignal pulses also in the at least one DPSK channel; interfering thefirst and the second optical signal pulses in an aligned state todemodulate the optical signal from having DPSK channels to an opticalsignal having channels modulated in intensity; outputting thedemodulated optical signal onto at least one main output optical path;and directing channels in the at least one main output optical path ontotributary optical paths.
 20. The method according to claim 19 furtherincluding converting the demodulated optical signal on the tributaryoptical paths into respective, corresponding, electrical signals. 21.The method according to claim 19 wherein demodulating the optical signalincludes interfering adjacent optical signal pulses.
 22. The methodaccording to claim 19 wherein demodulating the optical signal includesinterfering non-adjacent optical signal pulses.
 23. The method accordingto claim 19 wherein demodulating the optical signal includes interferingoptical signal pulses offset by a selectable number.
 24. The methodaccording to claim 19 wherein demodulating the optical signal includestuning an interferometer for receiving incoming wavelengths.
 25. Themethod according to claim 19 wherein wavelength spacing between carrierwavelengths defining the channels is an integer multiple of a channelrate of the optical signal.
 26. The method according to claim 19 whereinwavelength spacing between carrier wavelengths defining the channels isevenly divisible by an odd number of half channel rates of the opticalsignal.
 27. The method according to claim 26 further includingcorrecting polarity of the channels.
 28. The method according to claim27 wherein correcting the polarity of the channels includes correctingpolarity of electrical signals corresponding to the channels.
 29. Themethod according to claim 27 wherein correcting the polarity of thechannels includes optically correcting polarity of the channels.
 30. Themethod according to claim 19 wherein channel rate bit durations areinteger multiples of demodulating delay.
 31. The method according toclaim 19 further including feeding back signal(s) to transmitter(s) ofthe optical signal(s) to cause the transmitter to tune carrierwavelengths defining the channels for adjusting wavelength position orchannel separation.
 32. The method according to claim 19 furtherincluding optically amplifying the optical signal prior to demodulatingthe optical signal.
 33. The method according to claim 19 whereindirecting the channels onto the tributary optical paths includesfiltering the phase demodulated optical signal.
 34. The method accordingto claim 19 further including controlling the demodulatinginterferometer based on at least one pilot signal.
 35. The methodaccording to claim 19 used in an optical regenerator.
 36. The methodaccording to claim 19 used in a free space optic communications network.37. The method according to claim 19 used in a fiber opticcommunications network.
 38. An apparatus for demodulating an opticalsignal having Differentially Encoded Phase Shift Keying (DPSK) channels,the apparatus comprising: means for delaying first optical signal pulsesin the at least one DPSK channel of the optical signal by a multiple bitduration to align the first optical pulses with differentially encodedsecond optical signal pulses also in the at least one DPSK channel;means for interfering the first and second differentially encodedoptical signal pulses in an aligned state to demodulate the opticalsignal from having DPSK channels to an optical signal having channelsmodulated in intensity; means for outputting the demodulated opticalsignal onto at least one main output optical path; and means fordirecting channels in the at least one main output optical path ontotributary optical paths.
 39. The apparatus according to claim 38 whereinwavelength spacing between carrier wavelengths defining the channels isan integer multiple of a channel rate of the optical signal.
 40. Theapparatus according to claim 38 wherein wavelength spacing betweencarrier wavelengths defining the channels is evenly divisible by an oddnumber of half channel rates of the optical signals.
 41. The apparatusaccording to claim 40 further including means for correcting polarity ofthe channels.
 42. The apparatus according to claim 38 wherein channelrates are integer multiples of interferometer delay.
 43. A method fordemodulating an optical signal having Differentially Encoded Phase ShiftKeying (DPSK) channels, the method comprising: feeding back a signal toconstrain wavelength spacing of DPSK channels in an optical signal; andinterfering offset optical signal pulses in the optical signal todemodulate the optical signal from being DPSK modulated to beingintensity modulated.
 44. The method according to claim 43 furtherincluding directing channels in the demodulated optical signal from atleast one main optical path onto tributary optical paths.
 45. A systemfor demodulating an optical signal having Differentially Encoded PhaseShift Keying (DPSK) channels, the system comprising: means for feedingback a signal to constrain wavelength spacing of DPSK channels in anoptical signal; and means for interfering offset optical signal pulsesin the optical signal to demodulate the optical signal from being DPSKmodulated to being intensity modulated.
 46. The system according toclaim 45 further including means for directing channels in thedemodulated optical signal from at least one main optical path ontotributary optical paths.
 47. An apparatus for demodulating an opticalsignal having Differentially Encoded Phase Shift Keying (DPSK) channels,the apparatus comprising: a processor to feed back a signal to constrainwavelength spacing of DPSK channels in an optical signal; and aninterferometer that interferes offset optical signal pulses in theoptical signal to demodulate the optical signal from being DPSKmodulated to being intensity modulated.
 48. The apparatus according toclaim 47 wherein the interferometer directs channels in the demodulatedoptical signal from at least one main optical path onto tributaryoptical paths.
 49. An apparatus for demodulating an optical signalhaving Differentially Encoded Phase Shift Keying (DPSK) channels, theapparatus comprising: a delay line interferometer to (i) demodulate anoptical signal from having DPSK channels to an optical signal havingchannels modulated in intensity (ii) output the demodulated opticalsignal onto at least one main output optical path, and (iii) receive atleast one pilot signal to allow for stabilization and control of theinterferometer and wavelength alignment of the incoming opticalsignal(s); and channel selectors in the at least one main output path todirect channels onto tributary optical paths.